Potential Costs ofAcclimatization to aWarmer Climate: Growthof a Reef Coral with HeatTolerant vs. SensitiveSymbiont TypesAlison Jones1*, Ray Berkelmans21Centre for Environmental Management, Central Queensland University, Rockhampton, Queensland,Australia, 2Australian Institute of Marine Science, Townsville,Queensland, Australia Environmental Science Graduate Program CIAM 6117 Coastal Environment Abimarie Otaño
Introduction Coral reefs are vulnerable to climatic change. Coral survival rate depends on acclimatization to warmer conditions by shuffling symbiotic zooxanthellae algae, from thermal sensitive to a heat resistant genotype. To increase heat tolerance in a particular reef it must occur zooxanthellae community shift of multiple coral species. Coral depends on the symbiont energy1 to carry calcification process. External ion transport and eventually CaCo3 precipitation.1.Zooxanthellaefunction in the cora (rETRmax)l: Facilitates the nutrients needed for the secretion of calciumcarbonate skeleton. Produce 95% of the coral energy requirements through photosynthesis.
Scleractinian order Scleractinian are hard skeleton corals which polyps secrete a high rate of carbonate, distinguishing as the primary reef builder. Benefits: coastal protection, carbon sink, provides habitat for marine organism and touristic attractions. Growth rate of this species determines reef resilience and regeneration after extreme events (i.e. bleaching, hurricane impacts, anthropogenic pressures). Fast regeneration prevents phase shift directed to macro- algae and soft corals dominance. Study objectives: Determine physiological comparison between type C2 (thermal sensitive) and type D (thermal tolerant) symbiont function in Acropora millepora corals of the Keppel island in the Great Barrier Reef, Australia.
Scleractinian Acroporidae family4 genera, >160 species of Acropora in the Indo-PacificAcropora millepora: Near threatened species1 genera, 2 species of Acropora in the CaribbeanAcropora palmata Acropora cervicornisListed as Endangered specie in the 2006. (ESA, 2006).Acropora porifera
MethodsField study, at reef slope of Miall Island,NE Australia 43 pieces (15-20cm) of A. millepora colonies where cut and pruned to similar sizes. Symbiodinum genotyped with Single Stranded Conformational Polymorphism(SSCP) analysis of the algal nuclear ribosomal DNA. Only colonies with intense bands where selected. March 2004 and May 2006. Buoyant coral weight measurement, every 3 months from March to December 2005. Growth measurement experiment after bleaching event in February 2006. C2 and D colonies where placed in racks to allow recovery.
Results: Field study First experiment – before bleaching event in 2006 Growth rate of D colonies 38% lower that C2 (figure 3). Growth varied with season, 71% higher in spring than in winter (figure 4).Figure 3. Growth of A. millepora in the field Figure 4: Seasonal growth
Results: Field studySecond experiment- after bleaching event in February 2006. Gained half of buoyant weight (figure 5). Overall growth rate was 47% lower. Symbiodium was retained. Highest growth rate rate in spring (76% lower than 2005). Lowest growth rate in autumn and winter. Figure 5. Seasonal growth rate before and after bleaching.
Methods Laboratory Australian Institute of Marine Science 16 colonies where transplanted from Keppel Islands to Magnetic island to allow recovery and acclimatization. 6 explants (9 colonies type C2 and 7 type D) where cut and distributed in three tanks. Controlled temperature conditions 23oC (spring/autumn non- stressful) and 29oC (summer stressful conditions). Coral where fixed to plastic stand and rotated 180o daily to allow enough light exposure. Approximate natural diurnal light cycle: 3.5h shaded light, 5h un-shaded, 3.5h shaded and 12h darkness. Photosynthetically active radiation measurements F0 and Fm.Fluorumeter (Fv/Fm): to monitor health of explant after dark-adapted max yield, assessed each morning after 8 hours of darkness.
Methods: LaboratoryZooxanthellae densities and pigments: Explants where frozen (-20oC) and tissues where stripped with air gun. Volume was homogenized for 20s Zooxanthellae count on 8 independent drops with a compound light microscope. Centrifugation for 5s at 4oC separated algal pellet. Absorbance was measured with a spectrophotometer. Total Chlorophyll a was calculated from the equation of Jeffrey and Haxo.
Results: Laboratory study Buoyant weight gained in explants was 29% less in colonies with type D symbiont than in type C2 (figure 1). Zooxanthellae density for type D colonies was 22% lower. Zooxanthellae density at 29oC density was 21% lower than 23oC. Chlorophyll a in type D was 16% lower. Chlorophyll c2 in type D was 17% lower. Concentration of chlorophyll a and chlorophyll c2 at 29C was 20% and 19%, respectively, higher than 23C.
Figure 1. Growth rate in the laboratory Figure 2. Algal density and chlorophyll pigments
Points to discuss:1. In your opinion, corals zooxanthellae shift to thermal resistant genotype is beneficial or prejudicial to the reef community?2. What are the main environmental factors that might influence the laboratory and field studies? Which method provides the most reliable results?3. What can be done to ensure the coral reef diversity and functions taking into consideration climate change projections?
DiscussionGrowth rate is affected by:1. Symbiont shuffling to thermally tolerant type after thermal stress.2. Bleaching stress.✽Type D symbiodinium colonies had lower growth rate incomparison with type C2, even in non-stressful conditions.✽Shuffling to type D and C1 thermal tolerant symbiontocurred in A. millepora at Miall Island after bleaching event in2006.✽Coral growth reduced by 56% after bleaching.✽Acclimation by shuffling to thermal resistant symbiontreduce growth but improve heat tolerance and survival.
✽ Growth differed by a 50% in juvenile A. millepora between type D and C1 symbiont (Mieog et. Al. ).✽ rETRmax 87% higher in juvenile corals with type C1, correlates with a double 14C fixation.Lower rETRmax and growth in type D symbiont:✽ Result of the retaining of photosynthetically fixed carbon for metabolism and repair.✽ Increased use of energy for respiration.✽ Increased rate of photo-inhibition and reduces photosynthesis.
Conclusion Growth rate of A. millepora is affected by the shift to thermal-resistant zooxanthela. In the long term, heat tolerance and resilience benefits are much greater due to the expected climatic change. Further research is required to truly quantify the effect of symbiont genotypes on diverse coral growth as they acclimatize to climate change. Evidence is needed to determine if there exist a correlation between thermally sensitive symbiont and a reduce photosynthesis carbon fixation in other scleractinian corals.